A mortar shell fired by Sri Lankan troops struck a makeshift hospital killing 49 patients and staff, a government health official claims.

The attack, which also wounded 50 people, apparently hit the admissions ward of the area's only functioning medical facility and killed civilians and medical staff.

Around 430 people, including 106 children, are said to have died in the past two days as tens of thousands remain trapped in the war zone.

Meanwhile, troops have broken through Tamil Tiger defences as the government brushed off Western criticism of its continued assault.

In Geneva, United Nations humanitarian chief John Holmes said intransigence by both sides had created an "absolutely awful situation."

He said: "The LTTE are clearly still holding onto that population against their will, using them as human shields. The government have said they are not using heavy weapons. But the evidence suggests that they are continuing to do so, at least to some extent."

The two sides in Asia's longest modern war have blamed the other for the deaths of hundreds of civilians who have been killed since Sunday in artillery fire.

The claims are impossible to verify as the war zone is sealed off to most outsiders and those inside cannot be considered fully independent.

Sri Lanka's human rights minister Mahinda Samarasinghe insisted: "We are not using big guns, we are not using aerial bombardment, consciously to avoid civilian casualties."

He added: "Anyone who has common sense would know that in a conflict like this, there is bound to be some civilian casualties, but as long as it's proportionate under international law, that is permitted.

"But certainly not intentionally on the part of the government and certainly not by shelling because we are not shelling in to the no-fire zone."

The British, French and Austrian foreign ministers have all urged the UN's Security Council to take action to prevent more civilian deaths.

UN figures released in April showed nearly 6,500 civilians had been killed in three months this year, as the government drove the separatist rebels from their northern strongholds.

© Independent Television News Limited 2009. All rights reserved.